Yesterday I moved her into my birthing tank with the fry i already had.

now here's the confusing part... as its my day off work i did my water changes and stuff today... about an hour ago, and when i did this i checked to make sure she hadn't had babies yet... but now when i go back to check again, she's thin and no babies...

i know there's a possibility of eating the fry, but the fish already in that tank aren't interested in the fry, and i thought the birth would last a few hours...

Very simply she ate them. Sometimes Swordtails have only a few fry and sometimes they have many.

One way to avoid this is to get a breeding box (plastic box that has a slotted bottom to allow the fry to escape the female); place the female in the breeding box and keep the tank dark so she doesn't get stressed, then when she is done birthing put her back into your main tank.

The other alternative is to make sure your fry tank is heavily planted with plants that offer hiding places for the fry (i.e. Java moss, water wisteria, hornwort, anachris, dwarf hair grass). Sometimes floating plants like anachris, or hornwort or even a plastic leigh (Hawaiian necklace) will give the fry hiding places. I have used this method with some success as well as using the breeding box method with success (right now I have 17 sword fry that are almost a month old and are over an inch long).